Embodiments of the invention relate generally to a superconducting magnet assembly for magnetic resonance imaging (MRI), and more particularly to the protection of the superconducting magnetic assembly in the event of quenching of superconducting operation using an external dump resistor.
As is well known, magnet coils wound of superconductive material can be made superconducting when placed in an extremely cold environment. For example, magnet coils may be made superconducting when enclosed in a cryostat or similar pressure vessel containing a cryogen. The low operating temperature enables the superconducting wires of the magnet coils to be in a superconducting state, wherein the resistance of the wires is essentially zero. A power source may be connected to the coils for a period of time to ramp the current up or down through the coils, and the lack of electrical resistance in the coils enables current to continue to flow therethrough after the power source has been disconnected from the coils. Such constant current flow through the superconducting magnet without noticeable decay is known as a “persistent” mode of operation, which has seen wide application in a variety of fields, particularly MRI.
In a typical MRI magnet, the main superconducting magnetic coils are enclosed in a cryogen pressure vessel, which itself is contained within a vacuum vessel. An axial imaging bore is formed in the center of the vacuum vessel, wherein the main magnet coils produce strong magnetic field in the imaging volume of the axial imaging bore. A common cryogen utilized within the cryogen pressure vessel is liquid helium. During superconducting operation, liquid helium boils to form helium gas, which is either recondensed for recycling or vented to the atmosphere.
A primary concern in superconducting magnet assemblies is the discontinuance, or “quenching”, of the superconducting operation, which may produce undesirable voltages and temperatures within the magnet. A quench event occurs when an energy disturbance, such as from magnet coil frictional movement, heats a section of superconducting wire and raises the temperature of the superconducting wire above the critical level where the wire loses its superconducting state. The heated section of the wire becomes resistive, and the heating further raises the temperature of the section of wire and propagates to adjacent areas, thereby increasing the size of the normal section. Irreversible quench then occurs, wherein the electromagnetic energy of the magnet must either be quickly dumped or converted into thermal energy.
Sudden quenching of superconducting operation can cause sharp temperature rises, which can in turn damage the superconducting wires. In addition, the rapid decrease in the molecular density within the cryogen vessel resulting from such a sharp temperature rise reduces the ability of the cryogen gas to properly insulate the surrounding components, thereby resulting in possible voltage breakdown. Furthermore, the liquid helium or other cryogen used within the cryogen vessel rapidly becomes gaseous as the temperature within the cryogen vessel increases, and this gas (with rapidly increasing pressure) must be vented from the cryogen vessel, thereby resulting in the loss of a substantial amount of the costly cryogen. Such a quench also leads to significant downtime in the use of the MRI scanner, as the superconducting magnet must be given significant time to re-cool and re-ramp after quench.
Advantageously, after each quench of a superconducting magnet, the magnet usually shows progressive improvement in performance. This phenomenon, known as “training”, enables the magnet to settle into substantially constant performance after a series of training quenches such that the magnet eventually quenches at currents that are appreciably higher than the initial quench current. Accordingly, training quenches are a common phenomenon taking place during manufacture of superconducting magnet assemblies for MRI. The magnets are trained to above the operating current to decrease the likelihood of quenches occurring when the magnet assembly is operating in a persistent mode in the field. However, these training quenches still involve the undesirable loss of expensive cryogen, extensive system downtime, and potential component damage associated with conventional quenching, as discussed above. Selection of the magnet design parameters, specifically the fraction of critical current at which the magnet operates, depends on the magnet stability and amount of training quenches. More aggressive magnet designs that would operate closer to the critical current, employ less superconductor, and have lower cost would be enabled if the consequences of training quenches were eliminated or reduced.
It would therefore be desirable to have an apparatus and method capable of providing quench protection to a superconducting magnetic assembly without losing significant amounts of a cryogen, and without experiencing extensive system downtime to allow for magnet re-cooling.